Main content
Biology library
Course: Biology library > Unit 36
Lesson 1: Crash Course: Biology- Why carbon is everywhere
- Water - Liquid awesome
- Biological molecules - You are what you eat
- Eukaryopolis - The city of animal cells
- In da club - Membranes & transport
- Plant cells
- ATP & respiration
- Photosynthesis
- Heredity
- DNA, hot pockets, & the longest word ever
- Mitosis: Splitting up is complicated
- Meiosis: Where the sex starts
- Natural Selection
- Speciation: Of ligers & men
- Animal development: We're just tubes
- Evolutionary development: Chicken teeth
- Population genetics: When Darwin met Mendel
- Taxonomy: Life's filing system
- Evolution: It's a Thing
- Comparative anatomy: What makes us animals
- Simple animals: Sponges, jellies, & octopuses
- Complex animals: Annelids & arthropods
- Chordates
- Animal behavior
- The nervous system
- Circulatory & respiratory systems
- The digestive system
- The excretory system: From your heart to the toilet
- The skeletal system: It's ALIVE!
- Big Guns: The Muscular System
- Your immune system: Natural born killer
- Great glands - Your endocrine system
- The reproductive system: How gonads go
- Old & Odd: Archaea, Bacteria & Protists
- The sex lives of nonvascular plants
- Vascular plants = Winning!
- The plants & the bees: Plant reproduction
- Fungi: Death Becomes Them
- Ecology - Rules for living on earth
© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice
Evolution: It's a Thing
Hank gets real with us in a discussion of evolution - it's a thing, not a debate. Gene distribution changes over time, across successive generations, to give rise to diversity at every level of biological organization. Created by EcoGeek.
Want to join the conversation?
- When he is talking about us being x% similar to various other animals - how is this number determined?(21 votes)
- this number is determined by the percentage of DNA an animal has in common with another animal. For example when he speaks of Chimpanzees being 98.6% similar to humans, it's meant that 98.6% of a chimpanzees DNA is exactly the same DNA humans have.(26 votes)
- Why are you so sure we all are descendants from a single micro-organism. Why couldn't multiple "original" micro-organisms have formed and earths biodiversity be descended from this group of "original" micro-organisms. Who's to say entire orders now extinct couldn't have evolved from a plethora of these initial micro-organisms. What am I missing?(19 votes)
- While we do not know a great deal about what the last universal common ancestor (often called LUCA) must have been like, we can be fairly sure there was one and only one.
Please understand that referencing this organism doesn't mean a single individual, but rather a community of related organisms.
Whether there were multiple such species but only one lineage survived, I don't think we can say. But as far as everything that is alive today, and the various extinct ancestral species, we can be pretty confident in saying that they all have a single LUCA. There are many reasons for concluding this, including:
All living things use the same 20 amino acids, even though there are many more to choose from. It seems rather unlikely that independently evolving lifeforms would have selected exactly the same 20 amino acids (or for that matter, would have specifically had 20 and not 18 or 22 or some other number of amino acids).
Although there are some organisms with minor variations, all known organisms share the same basic genetic code. It is rather unlikely that independently evolving organisms would have happened to evolve exactly the same genetic code.
We could go on with many other features shared by all life that are rather unlikely to have evolved separately. So, it would seem that there must have been a LUCA about 3.5 billion years ago.
It should be pointed out that the LUCA could not have been the original life form. It clearly was not. The first forms of life would have been much more primitive.(23 votes)
- I thought that humans have about 50% of deoxyribonucleic acid as a banana, but is it also true with fruit flies, as mentioned at? 6:40(6 votes)
- Yes, bananas, apes, flies... they all have similar (to an extent) DNA!(4 votes)
- Why do some species evolve faster then others?(6 votes)
- humans can also push evolution along, antibiotics weeding out specific bacteria with resistance against said antibiotics can cause evolution.(2 votes)
- Would a virus be considered a living being?(3 votes)
- Viruses would not be considered living things, but their host cells, bacteria, would be considered a living thing.(4 votes)
- when was evolution first discovered?(2 votes)
- Charles Darwin was the first to put together the theory of evolution of species through natural selection, but others had previously observed evolution without fully understanding it. Dog breeders, for example.(3 votes)
- So does that mean in time we will evolve with some new feature too ? If yes then what it may be ?(2 votes)
- Yes! We could possibly evolve to become resisant to cancer or diabetes or other diseases, though they would have to take a long time.(1 vote)
- Were humans alive during the time of the dinosaurs? Or are we related to dinosaurs?(1 vote)
- Humans were not alive during the time on earth.
All life on earth is related, so we are related to dinosaurs, but the relationship is very very distant.(2 votes)
- what is meant by adaptive radiation(1 vote)
- in the March of Progress picture it depicts our evolution from monkeys to humans. i know we share a common ancestor but does it necessarily mean that we evolved from monkeys. I've read multiple explanations and theories and a majority were convincing, such as skull shrinkage being evident. i would like to hear your thoughts.(1 vote)
- No, we did not evolve from the monkeys that exist today.
We and the other apes, and the monkeys (and all life on earth) share common ancestors.
Our most recent common ancestor with the apes is more recent than our most recent common ancestor with the monkeys, which is why we are more closely related to the apes - we diverged from them more recently.(1 vote)
Video transcript
- Congratulations, this
is our last episode of our section on evolution and genetics, which puts us at the halfway
mark of Crash Course biology. So far, we've learned about DNA, genetics, natural selection, how cells multiply, population, speciation, replication, respiration, and photosynthesistation. I'm so proud of you, but I
couldn't let this section end without discussing the discussion that everybody can't help but
discuss these days, evolution. It's a thing, it's not a debate. Evolution is what makes life possible. It allows organisms to adapt to the environment as it changes. It's responsible for
the enormous diversity and complexity of life on Earth, which not only provides organisms with sources of food and
some healthy competition, it also gives us some truly
awesome stuff to marvel at. And even though evolution
makes living things different from one another, it also shows
us how we're all the same. All of life, every
single thing that's alive on the Earth today can claim
the same shared heritage, having descended from the
very first microorganism, when life originated on this
planet 3.8 billion years ago. There are people who will
say that this is all random, it's not, and that this clumsy process could not be possible for the
majestic beauty of our world. To them, I say, well, at least we agree that our world is beautiful, but, well, you're probably
not going to enjoy the rest of this video. To me, there are two sorts
of people in the world; those that are excited
about the power and beauty and simplicity of the
process of evolution, and those who don't understand it. And somehow, I live in
a country where only 40% of the population believes
that evolution is a thing. The only possible reason
for that, that I can accept, is that they just don't understand it. It's time to get real people. (upbeat music) First, let's understand what we mean when we talk about the
theory of evolution. Evolution is just the idea that gene distribution changes over time, which is an indisputable fact, which we observe all the
time in the natural world. But the theory of evolution
is a large set of ideas that integrates and explains
a huge mass of observations from different disciplines,
including embryology, paleontology, botany, biochemistry,
anatomy, and geophysics. In everyday language, the word theory means
hunch or even hypothesis, but in science, a theory is an idea that explains several phenomena at once. Thus, the theory of
evolution is a bunch of ideas that explain many things
that we, as humans, have observed for thousands of years. It's the theory that meticulously and precisely explains the facts and the facts are indisputable. So let's spend some time
going through the facts and how evolution
explains them all so well. First, fossils. The fossil record shows that
organisms that lived long ago were different from
those that we see today. Sounds obvious, but 200 years ago, it seemed a little bit crazy. When scientists started
studying dinosaur fossils in the 1820s, they
thought that all dinosaurs were basically giant iguanas. That's why the first fossil
dinosaur was named Iguanadon. It wasn't until the fossils
of two legged dinosaurs started showing up in the 1850s that scientists had to
grapple with the idea that organisms of the
past were somewhat similar to ones today, like
dinosaurs were to reptiles, but many of them took on a diversity that's barely recognizable to us. And all those ancient
not-really-iguanas were all extinct, either dying out completely or evolving into organisms
that survive today, like birds. Fossils make it clear that only evolution can explain the origin of
these new kinds of organisms. For instance, fossils taught
us that whales used to walk. Whales are cetaceans, a group of mammals that
includes porpoises and dolphins, and biologists long suspected that whales descended from land mammals, partly because some
modern whales still have the vestigial remnants of a
pelvis and hind limb bones. But it wasn't until recently,
the 1990s and 2000s, that the pieces really came together. First, paleontologist
discovered fossils of Dorudon, cetaceans that had different
skulls from modern whales, but still had the same
vestigial leg bones. Then they found even older fossil remains from another cetacean that
actually had hind legs and a pelvis, but the pelvis wasn't fused
to the backbone like ours is, so it did swim like a whale. But more importantly, it
still had ankle bones, and they were ankle bones that are unique to the order that includes
bison, pigs, hippos, and deer. So by following these clues left behind and fossilized bones, paleontologists were able to
track the origin of whales back to the same origin as bison and pigs. This leads us to another series of facts that evolution explains, not
how animals are different, but how they are incredibly similar. Last week, we talked about Carl Linnaeus an dhow he classified organisms by their structural similarities. Well, he didn't know anything
about evolution or genetics, but when he began grouping
things in this way, he hit upon one of evolution's most prominent clues,
homologous structures. The fact that so many organisms share so many finally detailed structures, shows us that we're related. Let's go back to the whale,
like my dog Lemon and me, the whale has two limbs
at the front of its body, its front flippers. And so does this bat, its wings. Inside our limbs, we all
have the very same structure. One longish bone on top, connected by two thin bones at the joint, followed by a cluster of small bones called the carpals, and
then our fingers or digits. We choose our forelimbs for
totally different purposes. The bat flies, the whale
swims, Lemon walks and I, you know, jazz hands. Building limbs like this
isn't the most efficient way to swim or fly or walk. Our limbs have the same structure because we descended from the same animal, something like this Morganucodon, which, yeah, had the
same forelimb structure. In the first stage of our existence, every vertebrate looks
almost exactly the same. Why? Because we're all descended from the same initial vertebrates. So yeah, our structures are
the same as other mammals and other vertebrates,
sure, but it also turns out that our molecules are the
same as like everything. In fact, if we were ever to
find life on Mars or something, the surefire way of knowing whether it's really extraterrestrial is to check and see if it has RNA in it. All living things on our
planet use DNA and or RNA to encode the information
that makes them what they are. The fact that we all use
the same molecule itself suggests that we are all
related, even if very distantly. But, what's more, by sequencing the DNA of any given creature,
we can see precisely how alike we are. The more closely related species are the more of the same
DNA sequences they have. So the human genome is 98.6% identical to that of a chimpanzee, our
closest evolutionary relative and fellow primate, but it's
also 85% the same as a mouse. And I wonder how you're
gonna feel about this. About half of our genes are
the same as in fruit flies, which are animals at least. So just as your DNA proves that you descended from your parents, your DNA also shows that you
descended from other organisms and ultimately, from that one
prokaryotic microorganism, 3.8 billion years ago, that
is the grandparent of us all. Now, when it comes to
species that are very similar to each other, like say, marsupials, their distribution around the
world or their biogeography is also explained extraordinarily well by the theory of evolution. Animals that are the most similar and are the most closely related, tend to be found in the same regions, because evolutionary change is driven in part by geographical change. As we talked about in
our speciation episode, when organisms become
isolated by physical barriers, like oceans or mountains, they take their own evolutionary courses. But the timescale that
we're talking about, the geographical barriers are much older and are often even the
result of continental drift. So marsupials, you know about marsupials, they can be found in many places, but they aren't evenly
distributed around the world. By far, the highest concentration
of them is in Australia. Even the majority of mammal fossils in Australia are marsupials. So why is Australia rife with kangaroos, koalas, and wombats, while
North America just has possums? Fossils show us that one of
marsupials earliest ancestors found its way to Australia
before continental drift turned it into an island
30 million years ago. More importantly, after
Australia broke away, placental mammals, like us,
evolved on the mainland mass and quickly out-competed most
of the marsupials left behind in what would become North
America and South America. So very few marsupials
remain in the Americas, while Australia has been drifting around, like some kind of marsupial Love Boat. Darwin's finches are another example of bio-geographical evidence, as he wrote in the Origin of Species. Darwin observed that
different species of finches on separate Galapagos Islands were not only similar to each other, but we're also similar to a species found on the South American mainland. He hypothesized that the island finches were all descendants
of that mainland finch and changed over time to be
more fit for their environments, a hypothesis that genetic
testing has since confirmed. Now, you'll remember, I hope, a few weeks ago, when I told you about Peter and Rosemary Grant,
the evolutionary biologists slash lovebirds who have
studied Galapagos finches since the 1970s. One of their greatest
contributions came in 2009 when studying finches on
the island of Daphne Major. They discovered that the
offspring of an immigrant finch from another island and
a Daphne Major finch had become a new species
in less than 30 years. This is just the latest
example of our fourth body of evolutionary evidence,
direct observation of evolution. Fact is, we have seen evolution take place in our own lifetimes. One of the fastest and most
common changes we observe is the growing resistance to
drugs and other chemicals. In 1959, a study of mosquitoes
in a village in India found that DDT killed 95% of mosquitoes on the first application. Those that survived reproduced and passed on their genetic
resistance to the insecticide. Within a year, DDT was
killing only 49% of mosquitoes and it continued to drop. The genetic makeup of the
mosquito population changed because of the selective pressures
caused by the use of DDT. But it's not just tiny
changes and tiny animals. We've also observed
larger animals undergoing some pretty striking changes. In 1971, for instance, biologists transplanted
10 Italian wall lizards from one island off the
coast of Croatia to another. 30 years later the immigrant
lizards' descendants have undergone some amazing
fundamental changes. Like even though the original lizards were mainly insect eaters, their digestive systems
had changed to help exploit the island's most abundant
food source, plants. They actually developed muscles between their large and small intestine that effectively created
fermenting chambers, which allowed them to
digest the vegetation, plus, their heads became
wider and longer to allow them to better bite and chew
the grasses and leaves. These are all great
examples of microevolution, a little frequency changes that happened rather quickly and in small populations. Macroevolution is just that microevolution on a much longer timescale. The sort of thing that
turns hippos into whales is a lot harder to observe for a species that, 200 years ago, thought
the dinosaurs were big iguanas. The part of the power of the human mind is being able to see far beyond
itself and the timescales that our own individual
lives are limited to. And I, for one, am pretty proud of that.